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A275287 Numbers n such that (26*10^n - 107)/9 is prime. 0

%I

%S 1,2,5,7,13,23,25,44,110,157,751,853,964,1135,2588,2602,2807,2888,

%T 3272,4046,4798,16720,21410,22285,25888,34586,51764

%N Numbers n such that (26*10^n - 107)/9 is prime.

%C For n>1, numbers n such that the digit 2 followed by n-2 occurrences of the digit 8 followed by the digits 77 is prime (see Example section).

%C a(28) > 10^5.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/primedifficulty.txt">Search for 28w77.</a>

%e 5 is in this sequence because (26*10^5 - 107)/9 = 288877 is prime.

%e Initial terms and primes associated:

%e a(1) = 1, 17;

%e a(2) = 2, 277;

%e a(3) = 5, 288877;

%e a(4) = 7, 28888877;

%e a(5) = 13, 28888888888877, etc.

%t Select[Range[0, 100000], PrimeQ[(26*10^# - 107)/9] &]

%o (PARI) lista(nn) = for(n=1, nn, if(ispseudoprime((26*10^n-107)/9), print1(n, ", "))); \\ _Altug Alkan_, Jul 22 2016

%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

%K nonn,more

%O 1,2

%A _Robert Price_, Jul 21 2016

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Last modified September 29 21:59 EDT 2020. Contains 337432 sequences. (Running on oeis4.)